ILS intercept

[John Citizen]

What is the correct way to join/intercept an ILS ?

Some pilots I fly with use the normal 3:1 descent profile to descend to their destination and then capture the ILS (LLZ and G/S) whilst still descending. (without ever levelling off)

Is this ok ? Or do we have to intercept /capture the LLZ before the G/S ? Must we have to capture the G/S from level flight from below the G/S ? Or is Ok to capture it on a continuous 3 degree descent beginning from flight levels ?

I am particularly interested in how the airlines do it or other aircraft which have steep descent profiles.

Thanks.

[Transition Layer]

Well technically you should intercept the LLZ before commencing descent on the G/S, and if the vectoring by ATC is good you will always achieve this. If flying a full procedure (i.e. an arc) the lead bearings/radials will have been designed for you to satisfy this requirement.

As for when to descend, well ideally you should intercept the G/S from below, but that doesn't have to mean you are level before intercepting. For noise abatement and fuel savings, its preferable (IMHO) to reduce your RoD and allow the G/S to drift down before intercepting.

For example, let's say you're in an aircraft that is grounding 140kts. To achieve a 3 degree G/S you'll need a RoD of roughly 700fpm (5 x 140). Let's say ATC clear you from 5000ft to 3000ft and give you a radar heading to intercept the LLZ and clear you for an approach. If you are below the G/S at this point, and you descend at greater than 700fpm, you'll never catch the G/S. Anything less than 700fpm and you will catch it at some point. So really it's up to you to ensure that you reduce your RoD sufficiently to ensure that you intercept the LLZ before the G/S. If you can achieve all of that without levelling off and having to open the taps, then it's a bonus!

Hope my drivel makes sense, i'm off to the pub for beers and 2up!

TL

[Chimbu chuckles]

It's preferable to capture from below but not mandatory.

You really should be on the localiser before descending on the glideslope.

In jets CDA (Constant Descent Approaches) are the most fuel efficient and in some places required for noise reasons, London Heathrow is an example where if you DONT achieve a low drag CDA your company will get a noise complaint.

Sometimes when I fly into LHR I end up on localiser intercept with the G/S bug a dot below me and sometimes a dot above me...I don't lose sleep over either but in a perfect world where I have juggled and guesstimated the ROD vs track nm perfectly I would be on intercept heading, LOC armed, with the GS just hovering above me. On final intercept and just before LOC comes alive I arm APP and the act of turning onto the LOC reduces my effective ROD a tad and I intercept both together...maybe LOC a few seconds before GS....and the thrust levers never left Flight Idle:ok:

[OzExpat]

As Chuck says, there's nothing that mandates the way that you achieve the intercept. The primary consideration is that you have some way to confirm that you're intercepting the real GP, rather than a false or erroneous one.

[Alistair]

Chuck,
The best bit then is watching the PF get the speed back from 300 KIAS and configure for a landing from 10 miles whilst descending on the glide :p or not as the case may be :O They just love the call 'xyz going around ' at LHR

[Chimbu chuckles]

Know what you mean...can't happen at LHR though because they have you back at 220KIAs leaving the hold...just gotta be in VS and be able to judge your track miles as they vector you all over the joint:ok: